Predistortion using FPGA

ABSTRACT

Predistortion logic for an optical communications laser or optical modulator, includes predistortion logic embodied in a field programmable gate array (FPGA). A first analog to digital converter (ADC) provides a representation of an RF signal at an input of the FPGA. A digital to analog converter provides a representation of an output of the FPGA to a laser or modulator.

PRIORITY CLAIM

The present application claims priority under 35 USC 119 to U.S.application No. 61/069,790 filed on Monday, Mar. 17, 2008, which ispresently pending, and which is incorporated herein by reference.

BACKGROUND

In cable television signal distribution systems, many separate radiofrequency (RF) television channels are simultaneously transmitted over atransmission line, and a required bandwidth thereof may extend from aslow as about 40 MHz to as high as about 1 GHz. Presently, cabletelevision distribution systems conventionally employ coaxial cable asthe transmission line for the RF spectra. Due to the significant signallosses inherent with coaxial cable transmission lines, repeateramplifiers must usually be provided at closely spaced distances, such asat every half mile along the extent of the network. These repeaters areexpensive, require a constant power supply, introduce cumulativedistortion, and must be maintained in order to provide continuous cableservice to subscribers.

Optical communications lasers, such as laser diodes and distributedfeedback (“DFB”) lasers, exhibit significant amounts of intermodulationdistortion, particularly composite second order (CSO) distortionproducts, i.e. distortion products of the type 2f₁, 2f₂, f₂−f₁, andf₂+f₁. Predistortion techniques have been proposed as one method forimproving distortion levels at the laser output.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, the same reference numbers and acronyms identifyelements or acts with the same or similar functionality for ease ofunderstanding and convenience. To easily identify the discussion of anyparticular element or act, the most significant digit or digits in areference number refer to the figure number in which that element isfirst introduced.

FIG. 1 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization.

FIG. 2 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization and an RF bypass.

FIG. 3 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization in which the high performance ADC in thefeedback path is replaced by a low-cost ADC and the input to this ADC isfiltered.

FIG. 4 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization in which the low-speed ADC is eliminated.

FIG. 5 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization in which, for very large distortions, thetransmitter may include an analog predistortion circuit in the mainsignal path (or in the driver for that matter) that significantlyreduces the distortion.

DETAILED DESCRIPTION

References to “one embodiment” or “an embodiment” do not necessarilyrefer to the same embodiment, although they may.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, refer to this application as awhole and not to any particular portions of this application. When theclaims use the word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list and anycombination of the items in the list.

“Logic” refers to signals and/or information that may be applied toinfluence the operation of a device. Software, hardware, and firmwareare examples of logic. Hardware logic may be embodied in circuits. Ingeneral, logic may comprise combinations of software, hardware, and/orfirmware.

Those skilled in the art will appreciate that logic may be distributedthroughout one or more devices, and/or may be comprised of combinationsof instructions in memory, processing capability, circuits, and so on.Therefore, in the interest of clarity and correctness logic may notalways be distinctly illustrated in drawings of devices and systems,although it is inherently present therein.

The techniques and procedures described herein may be implemented vialogic distributed in one or more computing devices. The particulardistribution and choice of logic is a design decision that will varyaccording to implementation.

Described herein are embodiments of logic and techniques to digitallycompute predistortion signals in field programmable gate arrays (FPGAs)and forward these to radio frequency (RF) components usingdigital-analog (DA) converters. The techniques employ fast FPGAs,analog-digital converters (ADCs), and digital-analog converters (DACs)technology. Applications include predistortion circuits than onlygenerate predistortion signals, and circuits that generate both the mainRF signal and predistortion signals. The input to the overall circuitcan be RF or digital. In case the input is RF, an AD converter may beused in front of the FPGA to digitize the RF signal spectrum that needsto be pre-distorted. Optionally an additional AD converter can be usedto digitize the overall system output (or a derivative thereof) tomonitor the overall predistortion effectiveness.

The difference between a system that processes only predistortionsignals and a system that processes the main signal as well issignificant because in most cases pre-distorter signals aresignificantly smaller than main RF signals. Requirements of AD and DAconverters and on internal computation bit-width are usuallysignificantly relaxed in systems that only generate pre-distortionsignals and have another path for the main RF signals.

Predistortion logic and techniques are described herein may be appliedto any type of RF signal generation, such as QAM generation, filteringand amplification. The FPGA and DA converter may provide both RF signaland predistortion signal components. Measurement and feedback of thesystem output is possible but not always required, depending on theamount of distortion compensation needed.

In other cases, such as RF amplifiers or fiber-optic transmitters, theFPGA and DA converter may preferably process predistortion signals. AnAD converter may be added to digitize the RF signal at the FPGA input.In such systems many types of distortion can be addressed such as laserdistortion, modulator distortion, fiber distortion, distortions due tofiber non-linearities and crosstalk or cross-modulation of multiplesignals in WDM systems. It should be noted that in these systems thedistortion to be compensated is usually fairly low, such as −50 to −60dBc relative to the signal level and the system noise floor is on asimilar level because low-cost RF predistortion methods can readilyachieve this performance. Only 10-20 dB of additional distortionreduction is desired. Because of this the required signal toquantization noise level of the DAC output is low and it can be expectedthat only 4-6 bits of data bus width are sufficient to achieve goodperformance. This modest requirement brings predistortion using FPGAswithin reach, even in relatively low-cost systems that operate at GHzbandwidths. Secondly it should be noted that with the use of modestmemory depth very complex distortion signals can be generated such asthey occur when signals from different sources (or even one source) aremixed together with different delays.

FIG. 1 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization. In this embodiment the RF signal isamplified and/or sampled by a high speed ADC 102. The samples areprocessed by an FPGA 104 which also adds predistortion signal componentsthat are intended to compensate distortion generated by lasers,modulators 110 and other system elements in a fiberoptic link. The DAC106 outputs to driver 106. The laser and/or modulator 110 output can bemonitored locally by a detector, amplifier 114 and a second ADC 112. TheFPGA 104 can use an algorithm to determine the distortion content of thefeedback signal. That information can then be used by the FPGA 104 toadjust the predistortion signals. The fact that the FPGA 104 receivesboth input and feedback signals allows to make a distinction betweendistortion components in the feedback signal and components that werealready present in the input signal. This configuration requires a highquality ADC 102 and DAC 106 because these may not affect the main signalthat passes through them.

FIG. 2 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization and an RF bypass. The main RF signalbypasses the FPGA 104, and a split-off signal reaches the FPGA 104. TheFPGA 104 generates predistortion components only that are provided tothe DAC 106. The ADC 102 must be good enough to facilitate generation ofthe predistortion components but does not need to support the mainsignal itself. This allows for a lower cost ADC 102. Secondly anEthernet input is shown on the FPGA 104. This digital input providesinformation that is transferred to a modulated signal by the FPGA 104and then is provided to the DAC 106. Thus non-analog channels can beadded to the transmitter output such as digital TV signals that are QAMmodulated. The DAC 106 must be of sufficient quality to support thesesignals, but still does not need to support the full analog signalquality.

FIG. 3 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization in which the high performance ADC 112 inthe feedback path is replaced by a low-cost ADC 112 and the input tothis ADC 112 is filtered. The filter 302 reduces the signal bandwidthinto this ADC 112 such that a lower rate ADC 112 can be used to sampleor sub-sample this signal. The FPGA 104 cannot anymore observe the fullspectrum of distortions (unless the filter is tunable) but it canobserve distortion bands selected by the filter 302. Because thedistortions caused by the laser/modulator 110 and optical system onlychange slowly in time and also are well behaved as a function offrequency the predistortion algorithm can maintain good distortioncancellation by interpolating between distortion observations in timeand at various frequencies.

FIG. 4 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization in which the low-speed ADC 112 iseliminated. Distortions at specific frequencies are provided to theinput ADC 102 in such a way that they do not interfere significantlywith the function of sampling the input signal by that ADC 102. Forinstance, distortions out of the signal information band (above 1 GHzand below 50 MHz for a practical analog fiber optic transmitter) can beprovided to the ADC 102. The predistortion algorithm can then cancelthese distortions and implicitly cancel distortions at other frequenciesby interpolation.

FIG. 5 is an illustration of an embodiment of an FPGA-based opticaltransmitter with linearization in which, for very large distortions, thetransmitter may include an analog predistortion circuit in the mainsignal path (or in the driver for that matter) that significantlyreduces the distortion. This also reduces the amount of predistortionsignal required from the DAC 106 and thus reduces requirements for DAC106, FPGA 104 signal path bit width (and thus FPGA 104 size) and ADC102.

Those having skill in the art will appreciate that there are variouslogic implementations by which processes and/or systems described hereincan be effected (e.g., hardware, software, and/or firmware), and thatthe preferred vehicle will vary with the context in which the processesare deployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a hardware and/orfirmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a solely software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes described herein may be effected, none of which isinherently superior to the other in that any vehicle to be utilized is achoice dependent upon the context in which the vehicle will be deployedand the specific concerns (e.g., speed, flexibility, or predictability)of the implementer, any of which may vary. Those skilled in the art willrecognize that optical aspects of implementations may involveoptically-oriented hardware, software, and or firmware.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood as notorious by those within the art that each functionand/or operation within such block diagrams, flowcharts, or examples canbe implemented, individually and/or collectively, by a wide range ofhardware, software, firmware, or virtually any combination thereof.Several portions of the subject matter described herein may beimplemented via Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs), digital signal processors (DSPs), orother integrated formats. However, those skilled in the art willrecognize that some aspects of the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in standard integratedcircuits, as one or more computer programs running on one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs running on one or more processors(e.g., as one or more programs running on one or more microprocessors),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and/or firmwarewould be well within the skill of one of skill in the art in light ofthis disclosure. In addition, those skilled in the art will appreciatethat the mechanisms of the subject matter described herein are capableof being distributed as a program product in a variety of forms, andthat an illustrative embodiment of the subject matter described hereinapplies equally regardless of the particular type of signal bearingmedia used to actually carry out the distribution. Examples of a signalbearing media include, but are not limited to, the following: recordabletype media such as floppy disks, hard disk drives, CD ROMs, digitaltape, and computer memory; and transmission type media such as digitaland analog communication links using TDM or IP based communication links(e.g., packet links).

In a general sense, those skilled in the art will recognize that thevarious aspects described herein which can be implemented, individuallyand/or collectively, by a wide range of hardware, software, firmware, orany combination thereof can be viewed as being composed of various typesof “electrical circuitry.” Consequently, as used herein “electricalcircuitry” includes, but is not limited to, electrical circuitry havingat least one discrete electrical circuit, electrical circuitry having atleast one integrated circuit, electrical circuitry having at least oneapplication specific integrated circuit, electrical circuitry forming ageneral purpose computing device configured by a computer program (e.g.,a general purpose computer configured by a computer program which atleast partially carries out processes and/or devices described herein,or a microprocessor configured by a computer program which at leastpartially carries out processes and/or devices described herein),electrical circuitry forming a memory device (e.g., forms of randomaccess memory), and/or electrical circuitry forming a communicationsdevice (e.g., a modem, communications switch, or optical-electricalequipment).

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use standard engineering practices to integrate suchdescribed devices and/or processes into larger systems. That is, atleast a portion of the devices and/or processes described herein can beintegrated into a network processing system via a reasonable amount ofexperimentation.

The foregoing described aspects depict different components containedwithin, or connected with, different other components. It is to beunderstood that such depicted architectures are merely exemplary, andthat in fact many other architectures can be implemented which achievethe same functionality. In a conceptual sense, any arrangement ofcomponents to achieve the same functionality is effectively “associated”such that the desired functionality is achieved. Hence, any twocomponents herein combined to achieve a particular functionality can beseen as “associated with” each other such that the desired functionalityis achieved, irrespective of architectures or intermedial components.Likewise, any two components so associated can also be viewed as being“operably connected”, or “operably coupled”, to each other to achievethe desired functionality.

1. Predistortion logic for an optical communications laser or opticalmodulator, comprising: predistortion logic embodied in a fieldprogrammable gate array (FPGA); a first analog to digital converter(ADC) providing a representation of an RF signal at an input of theFPGA; and a digital to analog converter providing a representation of anoutput of the FPGA to the laser or modulator.
 2. The predistortion logicof claim 1, further comprising: the RF signal also provided to the laseror modulator by bypassing the FPGA.
 3. The predistortion logic of claim1, further comprising: an Ethernet input to the FPGA.
 4. Thepredistortion logic of claim 1, further comprising: a feedback path fromthe output of the laser or modulator to the input of the FPGA.
 5. Thepredistortion logic of claim 4, further comprising: the feedback pathcomprising a second ADC.
 6. The predistortion logic of claim 4, furthercomprising: the feedback path comprising a filter.
 7. The predistortionlogic of claim 1, further comprising: a feedback path from the output ofthe laser or modulator to the first ADC.
 8. The predistortion logic ofclaim 7, further comprising: the feedback path comprising a filter. 9.The predistortion logic of claim 2, further comprising: the RF signalalso provided to the laser or modulator by bypassing the FPGA via ananalog linearizer.
 10. A system comprising: an optical communicationslaser; predistortion logic embodied in a field programmable gate array(FPGA); a first analog to digital converter (ADC) providing arepresentation of an RF signal at an input of the FPGA; and a digital toanalog converter providing a representation of an output of the FPGA tothe laser.
 11. The system of claim 10, further comprising: the RF signalalso provided to the laser by bypassing the FPGA.
 12. The system ofclaim 10, further comprising: an Ethernet input to the FPGA.
 13. Thesystem of claim 10, further comprising: a feedback path from the outputof the laser to the input of the FPGA.
 14. The system of claim 13,further comprising: the feedback path comprising a second ADC.
 15. Thesystem of claim 13, further comprising: the feedback path comprising afilter.
 16. The system of claim 10, further comprising: a feedback pathfrom the output of the laser to the first ADC.
 17. The system of claim16, further comprising: the feedback path comprising a filter.
 18. Thesystem of claim 11, further comprising: the RF signal also provided tothe laser by bypassing the FPGA via an analog linearizer.